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Apple today released an update for Snow Leopard that warns users that they've downloaded fake Mac security software and claims to scrub machines already infected with the so-called "scareware."

Chet Wisniewski, a security researcher with U.K.-based Sophos, confirmed that the update alerts users when they try to download any of the bogus MacDefender antivirus software.

Wisniewski had not yet tested the malware cleaning functionality of the update, but was confident that it would work.

"It's reasonably trivial to remove MacDefender," said Wisniewski, using the name for a growing family of scareware. "It's not burying itself in the system, not compared to some of some of the crap that we see on Windows."

Tuesday's update, labeled 2011-003, adds a new definition to the rudimentary antivirus detection engine embedded in Mac OS X 10.6, aka Snow Leopard, and also increases the frequency with which the operating system checks for new definitions to daily.

Before today, Apple had added only five detection signatures to the antivirus component of Snow Leopard.

If a user downloads a MacDefender variant using Safari, iChat or Mail -- the Mac's native email client -- Snow Leopard posts an on-screen notice that reads, "filename will damage your computer. You should move it to the Trash."

The default button of "Move to Trash" will dump the malware into the bin.

A week ago, Apple publicly acknowledged the threat posed by what security experts call "scareware" or "rogueware," and promised to issue an update to detect and delete such software.

The terms refer to bogus security programs that claim a personal computer is heavily infected with worms, viruses and other malware. Once installed, scareware nags users with pervasive pop-ups and fake alerts until they fork over a fee to purchase the worthless program. MacDefender demands $60 to $80 as its "registration" fee.

French security company Intego reported MacDefender, the first piece of professional-looking scareware to target Macs, in early May. Since then, several variants have appeared, including one named MacGuard that installed without requiring a password.

Apple's update was offered only to customers running Snow Leopard; Macs powered by the older Mac OS X 10.5, known as Leopard, will not receive the same anti-MacDefender protections.

Mac users can download the update manually from the Apple site, or install it using the operating system's integrated update service.

After Tuesday's update, Snow Leopard detects Mac scareware and warns the user to dump the file in the Trash. (Image: Sophos)